Process of washing and protecting photographic silver images



United States Patent 3,529,962 PROCESS OF WASHING AND PROTECTING PHOTOGRAPHIC SILVER IMAGES Howard C. Haas, Arlington, Mass, assignor to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware No Drawing. Filed Sept. 15, 1965, Ser. No. 487,597 Int. Cl. G03c 5/54, 5/26 US. CI. 96-29 9 Claims ABSTRACT OF THE DISCLOSURE The stability of silver images produced by diffusion transfer photographic techniques is enhanced by applying to the silver print a solution of an aromatic or aliphatic carboxylic acid substituted partial acetal of polyvinyl alcohol and drying the solution to form a protective coating thereon. Preferred compositions contain, in addition to the polymer, a salt of a heavy metal which forms an insoluble sulfide, water, a water-miscible organic solvent and a volatile alkaline material.

This invention relates to a composition of matter comprising a novel mixture of materials, and more particularly to the washing and protecting of a photographic silver image by applying to at least one of its faces in a layer a composition containing water for dissolving residual water-soluble processing reagents from the image and a novel mixture of materials for producing a protective coating on the image when the composition is dried.

Objects of the present invention are: to provide a process for increasing the stability of a photographic silver image with a novel composition comprising water for washing the image when the composition is applied and a solution of such materials for forming a coating upon the image when the composition is dried; to provide, as a novel product, a protected photographic image produced by such a process; to provide, for use in a process of the foregoing type, a composition comprising a carboxylic acid substituted partial acetal of polyvinyl alcohol; and to provide, for use in a process of the foregoing type, a composition comprising a polymer of the name group and a salt of a heavy metal which forms an insoluble sulfide.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation and order of one or more of such steps with respect to each of the others, and the composition and product possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the appended claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description.

The process of the present invention is particularly useful for washing and protecting thin photographic silver images prepared in accordance with well known diffusion transfer processes, for example, as described in US. Pat. No. 2,719,791, issued to Edwin H. Land on Oct. 4, 1935. An extremely thin, photographic silver image of the foregoing type ordinarily retains traces of the photographic reagents with which it has been processed, and the continued presence of which may adversely affect its stability. For example, silver may be oxidized by sulfur from the residue of sodium thiosulfate which has been employed as a solvent. Or, the silver may be oxidized by numerous oxidizing agents present in the atmosphere. Also, the image may comprise traces of silver ion in equilibrium with the "ice metallic silver; where the image is subjected to materials which form insoluble silver residues, such as sulfides, they react with the silver ion, causing more metallic silver to shift to the ionic state, ultimately detroying the silver image entirely. Furthermore, traces of unoxidized developer, if oxidized by atmospheric oxygen, may discolor the highlights of the image.

It has been proposed, in order to improve the stability of such an image, to coat it with a composition comprising an aqueous solution of a film-forming material. The water acts to wash traces of the photographic reagent from the image and to so distribute the film-forming material that it provides the image with a protective coating when the dispersion is dried.

Processes of the foregoing type are more fuly described in US. Pat. No. 2,719,791, issued Oct. 4, 1955 to Edwin H. Land for Method of Improving the Stabilization of 'Finished Photographic Prints by Applying A Liquid Composition Thereto.

The compositions of matter contemplated by the present invention generally comprise mixtures of compatible components which together possess properties not possessed by the components themselves. These components include an aromatic 0r aliphatic carboxylic acid substituted partial acetal of polyvinyl alcohol and preferably, a salt of a heavy metal which forms an insoluble sulfide and a volatile alkaline material, such as ammonia. The polymeric material functions primarily to impart to the composition the following unusual combination: substantial impermeability to elemental sulfur and hydrogen sulfide, for example from sulfur-containing materials with which the composition may come into contact; image stability throughout Wide temperature and humidity ranges; resistance to generalized oxidation of silver; resistance to the effects of prolonged solar radiation; optical clarity; and non-tackiness. The heavy metal salt in conjunction with the polymeric material functions primarily to impart to the composition excellent resistance to penetration by sulfides, for example, from the atmosphere, abrasion resistance, and, further, contributes to the nontackiness of the composition.

In accordance with the present invention, a preferred composition for this purpose comprises a solution prepared from: (1) a salt of a heavy metal which forms an insoluble sulfide; (2) water; (3) a water-miscible organic solvent; (4) a volatile alkaline material and (5) a polymer composed of the recurring unit having the formula:

(I) H 0 OH l t R where R is selected from the group consisting of and m is an integer from 1-4, inclusive.

As examples of polymeric materials within the scope of the invention, mention may be made of polymers composed of the recurring unit of:

(1) Phthalaldehydic acid partial acetal of polyvinyl alcohol,

[-CHz-CH-CHz-OH-OHz-CH] l o ($112M (2) 4-formyl butyric acid partial acetal of polyvinyl alcohol (3) 2-formyl acetic acid partial acetal of polyvinyl alcohOlt This solution, because of its aqueous character, is readily adapted to dissolve residual water-soluble photographic processing reagents from a photographic image.

The polymeric materials of this invention may be prepared by reacting a low molecular weight polyvinyl alcohol, preferably one which is completely hydrolyzed, with a compound having the formula RCHO, where R has the same meaning as is given above, in the presence of an acid catalyst.

Alternatively, the polymers may be prepared by reacting polyvinyl alcohol with an ester having the formula:

oonom-o 0 O-R o O-R where m is an integer from 14, inclusive, and each R is an alkyl radical, or the corresponding acetals of the above esters, and subsequently hydrolyzing the ester group to the corresponding acid. In the preferred polymeric formulations, a total of 28% to 42% of the hydroxyl groups on the polyvinyl alcohol chain are substituted with the acid function groups. It has been found that if the substitution is as low as 20%, the resulting composition may be undesirably viscous. The substitution can theoretically be as high as about 90% but there are substantially no advantages to be gained thereby.

It is desirable that the composition contain a salt of a heavy metal which forms an insoluble sulfide in a concentration sufficient to provide the coating formed on the face of the image when the composition is dried with the quantity of salt which is large relative to the quantity of the silver in the image. Consequently, atmospheric contaminants, e.g., sulfides which penetrate the coating react with the heavy metal salt in preference to the silver ion of the image as discussed above. Preferably, the concentration of the heavy metal salt, by total weight of the preferred composition, ranges approximately from 1% to 15%. The heavy metal salt, preferably alkali soluble, may contain a heavy metal cation such as zirconium or zinc, and an anion such as acetate, sulfate, nitrate and formate; heavy metal salts which are pale in appearance and which react to form pale sulfides are preferred. The salts of zinc in particular are preferred because they and their sulfides are white. The quantity of salt, although large relative to the quantit of silver in the image, should be so small that the salt and its sulfide do not appreciably affect the appearance of the image.

In one embodiment, the composition contains zinc oxide as the salt of a heavy metal and ammonia as the alkaline material. It has been found that when ammonia and zinc oxide are used in conjunction with the polymeric materials described herein, a particularly non-tacky, hard coating is formed. It is believed that the zinc oxide and ammonia react to form a zinc ammonium complex which complex, in turn, enters a state of equilibrium with zinc hydroxide and ammonia. When the composition is applied as a coating to photographic images, ammonia evaporates, generating additional zinc hydroxide. As zinc hydroxide is formed, however, it reacts with the available carboxylic acid groups on the polymer chain in such a way as to cross-link the materials.

It is believed that the ultimately formed cross-linked polymers are composed of the recurring unit having the formulation:

The use of zinc oxide in the presence of ammonia as a cross-linking agent for acidic polymeric materials in coating compositions is further described and claimed in copending application Ser. No. 487,596, filed concurrently herewith.

The organic solvent, examples of which are low molecular weight alcohols such as methanol, ethanol and isopropanol, dioxane and low molecular weight ketones such as acetone and methylethyl ketone, and the base, preferably weak, an example of which is ammonia, cooperate to cause the polymer, which is ordinarily insoluble in water alone, to dissolve. The water-miscible, organic solvent functions addditionally to impart to the solution an overall, quick-drying character.

While in the preferred embodiment of the composition described herein, ammonia is the alkaline material of choice, other commercially available amines also provide satisfactory coating ingredients, the purpose of such materials being to dissolve the acidic polymeric material and to impart to the final composition a pH of at least about 8, and preferably, about 9 or higher. The use of alkaline print-coating compositions is further described and claimed in copending application Ser. No. 487,578 filed concurrently herewith.

It is also sometimes desirable to add an additional polymeric material to the coating composition to further enhance its protective properties. The preferred material for this purpose is a hydantoin formaldehyde condensation polymer, such as that described in US. Pat. No. 2,874,045, issued Feb. 17, 1959 to Edwin H. Land, for Process of Washing and Protecting Photographic Silver Images.

The following nonlimiting examples provide illustrations of the methods in which the polymers described herein may be synthesized.

EXAMPLE 1 Phthalaldehydic acid partial acetal of polyvinyl alcohol polymer composed of the recurring unit 88 grams (2 moles) of low viscosity, low molecular weight polyvinyl alcohol having less than 1% acetate were dissolved in 500 cc. of water and cooled. 150 grams (1 mole) of hthalaldehydic acid were added with stirring and the mixture was heated at 50 C. under nitrogen. 1 cc. of concentrated sulfuric acid was added. The polymer separated as a soft, rubbery mass. After four hours, the liquid was decanted and the product washed in hot water. It was further purified by precipitation from ethanol water (containing 1 gram of potassium acetate) into water, washed in water and dried. The final product was predominantly the phthalaldehydic acid partial acetal of polyvinyl alcohol, with a small amount of the corresponding phthalide; the composition was confirmed by infrared analysis.

EXAMPLE 2 4-formyl butyric acid partial acetal of polyvinyl alcohol polymer composed of the recurring unit 4.4 grams (0.1 mole) of polyvinyl alcohol were added to a mixture of 14.4 grams of ethyl-4-formyl butyrate and 40 ml. of glacial acetic acid. 0.5 gram of p-toluene sulfonic acid was added and the mixture stirred at 60 C. for about 18 hours. The polymer, ethyl-4-forrnyl butyrate partial acetal of polyvinyl alcohol, was isolated by precipitation into ether and purified by reprecipitation from methanol into ether and dried under vacuum at 45 C. The thus-prepared polymer was hydrolyzed by dissolving it in 50 ml. of dioxane and adding 50 ml. of 5% aqueous sodium hydroxide. The mixture was heated on a steam cone for about one hour, after which dilute (5%) hydrochloric acid was added slowly until all the polymer had precipitated. The resulting pH was 6. The polymer was washed in water and precipitated from methanol into water and dried at 40 C. under vacuum. 3 grams of dry polymer were recovered.

EXAMPLE 3 3-formyl propionic acid partial acetal of polyvinyl alcohol polymer composed of the recurring unit 4.4 grams (0.1 mole) of polyvinyl alcohol were added to a mixture of 13.0 grams of ethyl-3-formyl propionate and 40 ml. of glacial acetic acid. 0.5 gram of p-toluene sulfonic acid was added and the mixture stirred at 60 C. for about 18 hours. The resulting polymer, ethyl-3- formyl propionate partial acetal of polyvinyl alcohol, was isolated by precipitation into ether and purified by reprecipitation from methanol into ether and dried under vacuum at 45 C. This polymer was hydrolyzed by dissolving it in 50 ml. of dioxane and adding 50 ml. of 5% aqueous sodium hydroxide. The mixture was heated on a steam cone for about one hour and dilute (5%) hydrochloric acid was added slowly until all the polymer had precipitated and the pH was 6. The polymer was washed in water and precipitated from methanol into water and dried at 40 C. under vacuum. 3.5 grams of dry polymer were recovered.

The preferred coating composition may be prepared by dissolving the polymeric materials in the water, organic solvent and base at a temperature ranging from room temperature to about 40 C. under a reflux condenser to prevent evaporation. Thereafter, the heavy metal salt is added. A preferred composition comprises the foregoing ingredients in the following proportions:

Acid polymer825 grams.

Hydantoin formaldehyde condensation polymer-5-30 grams.

Salt of heavy metal having insoluble sulfide1-10' grams.

Water-5070 cc.

Organic solvent-30 50 cc.

Base-suificient to dissolve the polymer and give a final pH of about 9.

The following examples demonstrate the preparation and eifectiveness of photographic coatings comprising the above compositions:

EXAMPLE 4 A composition for washing and protecting photographic silver images was prepared by combining and blending the following materials to provide a uniform solution:

Phthalaldehydic acid partial acetal of polyvinyl alcohol Dimethyl hydantoin formaldehyde condensation polymer 13.5 g.

Zinc acetate3.24 g.

Water--50 cc.

Isopropyl alcohol-30 cc.

Ammonia (29.4% aqueous solution)10 cc.

The final pH was about 9.

EXAMPLE 5 A composition was washing and protecting photographic silver images was prepared by combining and blending the following materials to provide a uniform solution:

Phthalaldehydic acid partial acetal of polyvinyl alcohol Dimethyl hydantoin formaldehyde condensation polymer13.5 g.

Zinc oxide-1.44 g.

Water50 cc.

Isopropyl alcohol30 cc.

Ammonia (29.4% aqueous solution)-10 cc.

The final pH was about 9.

Silver transfer images were prepared by exposing photosensitive silver halide elements to a step-wedge, and developing the latent images formed thereby by spreading between the exposed photosensitive elements and superposed image-receiving elements a processing composition containing a viscous aqueous solution of a silver halide developing agent, a silver halide solvent, and an alkali. The respective elements were maintained in superposed relation for a predetermined period, during which time the exposed silver halide was reduced to silver in the photosensitive elements. The unreduced silver halide formed a water-soluble complex silver salt which diffused through the layer of composition to the image-receiving elements where, upon being reduced to silver, it formed a silver print. At the end of the predetermined period, the photosensitive elements, together with the layer of composition, were stripped from the image-receptive elements.

The photographic silver images so prepared were swabbed by means of an aborbent applicator with the solutions of Example 4 or 5. The photographic prints so treated dried rapidly under ordinary atmospheric conditions, after which they were subjected to the following tests:

(a) The prints were allowed to remain in closed chambers at a temperature of 100 F. and 80% relative humidity for a period of about 40 hours.

(b) The prints were suspended over Water at 100 F. for a period of about ten days.

(c) The prints were subjected to dry air at 100 F. and 20% relative humidity for a period of about 16 hours, to test for cracking of print layer.

(d) The prints were subjected to 4 and 8 hour periods in an atmosphere containing hydrogen sulfide (provided by an aqueous solution of sodium sulfide) at room temperature.

(e) The prints were stacked face-to-face at relative humidities of 20% and 80%; a weight was placed on the stack.

After all of the foregoing tests, it was observed that in none of the prints was there any deterioration of the silver image, nor was there any loss in density of the light steps of the image. The prints were non-tacky, as evidenced by the fact that those treated with the composition of Example 4 exhibited only the slightest tendency to adhere to one another in test (e) above, and those treated with the composition of Example 5 did not tend to adhere to all. Moreover, in the prints treated with the composition of Example 5 there was no image discoloration; there was only very slight image discoloration of the prints treated with the composition of Example 4.

Examples of photographic materials useful in the production of the photographic sliver images to which the preferred composition may be most advantageously applied are described in detail in Patent No. 2,543,181, issued in the name of Edwin H. Land on Feb. 27, 1951, for Photographic Product Comprising a Rupturable Container Carrying a Photographic Processing Liquid, and in Pat. No. 2,647,056, issued in the name of Edwin H. Land on July 28, 1953, for One-Step Photographic Transfer Process.

Preferably, the composition of the present invention is applied to a photographic silver image to be washed and protected by means of an absorbent applicator composed, for example, of flannel, cotton batting, or cellulose sponge charged with the composition. When the face of the image is swabbed with such an applicator, residual reagents in the image are dissolved and, for the most part, transferred into the applicator, and the image becomes coated with a thin layer of the dispersion. The dispersion can then be dried to form a protective coating.

The preferred composition referred to above, after being swabbed by means of such an absorbent applicator onto a thin photographic silver image of the above-described type, rapidly dries under ordinary atmospheric conditions to form a coating having excellent protective properties for the image.

Since certain changes may be made in the above process, composition and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a process for washing and protecting a diffusion transfer photographic silver print containing a residue of the reagents with which it has been processed, the steps of applying to one face of said print a thin layer of a composition comprising a salt of a heavy metal which forms an insoluble sulfide, water, a water-miscible organic solvent, an alkaline material, and a film-forming polymer composed of the recurring unit having the following formulation:

where R is selected from the group consisting of 5 coon and (CH COOH, and m is an integer from 1-4, inclusive, said composition has a pH of at least 8; and drying said layer to form a protective coating on said print.

2. The process of claim 1 wherein said polymer is a phthalaldehydic acid substituted partial acetal of polyvinyl alcohol having a total of 28 to 42% of the hydroxy groups on the polyvinyl alcohol chain substituted with the acid-substituted acetal groups.

3. The process of claim 1 wherein said alkaline material is ammonia. 4. The process of claim 1 wherein said salt of a heavy metal is zinc acetate.

5. In a process for washing and protecting a photographic silver diffusion transfer print, the step of applying to one face thereof in a thin layer a composition comprising a solution prepared from a film-forming polymer of a carboxylic acid partial acetal of polyvinyl alcohol composed of the recurring unit having the formula:

(-OHz-(|JHCH2CHCHz-OH) n1 l I R where R is selected from the group consisting of COOH and (CH COOH and m is an integer from 1-4, inclusive; water and a water-miscible organic solvent, said composition has a pH of at least 8, and the step of forming a protective coating on said print by drying said layer to produce a residue composed primarily of said carboxylic acid partial acetal of polyvinyl alcohol.

6. The process of claim 5 wherein said carboxylic acid partial acetal of polyvinyl alcohol is a phthalaldehydic acid partial acetal of polyvinyl alcohol having a total of 28% to 42% of the hydroxy group on the polyvinyl alcohol chain substituted with acid function.

7. The process of claim 5 wherein said solution includes a salt of a heavy metal.

8. The process of claim 5 wherein said solution includes zinc acetate.

9. The process of claim 5 wherein said solution includes ammonia.

References Cited UNITED STATES PATENTS 2,830,900 4/1958 Land et al. 9629 2,874,045 2/ 1959 Land 9629 2,956,877 10/1960 Land et al. 9629 2,979,477 4/ 1961 Land 26029.4

3,174,858 3/1965 Van Hoof et al. 9629 3,325,283 6/1967 Barstow et al. 963

I. TRAVIS BROWN, Primary Examiner M. F. KELLEY, Assistant Examiner U.S. Cl. X.R. 

